Here is internet image of TR6 flywheel fitted.      Marco, I think how far the ring gear of TR 6 is from the rear engine plate is the question also.  It is the biggest diameter you need to get into the MX5 gearbox bell housing. An accurate measurement is needed. We know the MX5 gearbox goes in Vitesse  as someone gave photos of it in your article about fitting the replacement gearbox.   Vitesses uses a flywheel the same ring gear as TR6.    It was YOW500 who posted the images in